T5 uvb proper distance?

[CastielTheBeardy]

I feel like i ask too many questions lol but im just curious how far it should be, not the basking area, its for the 2/3 of the tank for the T5. It is placed inside my tank the basking bulb is a mercury vapor bulb the right side has no lights and the small dome is for a CHE

 

[Sadie]

You need a piece of decor directly under the UVB 12-15 inches --- as it sits right now shes not getting much or any at all from the way I see it -- is it in a fixture w/ a reflector?